A resonance oscillator of this type is already known from FR-A-374 934. The known resonance oscillator is designed for operations in sea water. As the resonant chamber, it has a cylindrical body which is open on one end and is connected with the surrounding region via openings formed in its holder. The resonant chamber is not filled with a sealed-off liquid volume. Rather, it is filled with the same liquid that is to be sounded. Further, this prior art reference discloses as an acoustic pick-up a cylindrical part attached inside onto the ship's wall, the base thereof, via an outside thread provided at the base, being screwable to an inside thread of the cylindrical pick-up. Also in this case, the resonant chamber is connected with the ambient sea water through an opening in the ship's wall and does not form a sealed-off liquid volume. Thus, in the prior art the density values of sea water and the sound velocity in sea water are effects which have to be accepted from the beginning. Moreover, resonance adaptation takes place only on the side of the receiver while on the side of the transmitter no change in the length of the resonant chamber is provided at all. The application range therefore is restricted.